This invention relates to systems in which data is represented by signals having periodic components.
A binary frequency shift-keyed modulation scheme, for example, may represent a data bit by one of two pre-established frequencies: a "mark" frequency for a bit having a "1" value; and a "space" frequency for a "0" valued bit. The mark and space frequencies may be generated by an analog voltage controlled oscillator, and their values are chosen to be within the passband of the channel
When the channel is a telephone line and the information is being sent to a computer, techniques are often used to detect unauthorized access to the computer.
For example, The person seeking access may be required to send a simple password, or to encrypt the information bits before transmission, or to send a separate identification bit stream. Alternatively the receiving device may dial back the sender to confirm that the planned access is authorized.
Where the channel bandwidth is adequate, spread spectrum systems may be used. The original signal to be sent is first modulated by a pseudo-random masking sequence to include many more frequency components than the original signal. The additional frequency components are spread out in the vicinity of the center frequency of the original signal. The receiver performs the inverse of the pseudo-random masking sequence on the received signal to recover the original signal.